1. Field of the Invention
This invention relates to a process for reducing the residual content of free alkylating agent in aqueous solutions of amphoteric or zwitterionic surfactants by aftertreatment of the solutions with ammonia, an amino acid containing 2 to 8 carbon atoms or an oligopeptide.
2. discussion of Related Art
Amphoteric and zwitterionic surfactants are characterized by a molecular structure which contains two functional groups of different polarity, generally a cationic ammonium group and an anionic carboxylate or sulfonate function. Surfactants having an amphoteric or zwitterionic structure combine excellent cleaning power with good dermatological compatibility. In addition, since they form a dense stable foam in aqueous solution which does not collapse even in the presence of soap, these classes of surfactants represent valuable raw materials for the production of personal hygiene preparations, particularly hair shampoos and shower baths.
Amphoteric or zwitterionic surfactacts are synthesized from primary, secondary or tertiary amino compounds which are reacted with a suitable alkylating agent [J. Falbe (ed.), "Surfactants in Consumer Products", Springer Verlag, Berlin/Heidelberg, 1987, pages 114 et seq.].
Thus, alkyl betaines or alkyl amidobetaines of amphoteric structure are obtained, for example, from the quaternization of tertiary amines [Seifen-Ole-Fette-Wachse, 108, 373 (1982)]or fatty acid amidoamines [Parf. Kosm. Arom. 75, 67 (1968)]with halocarboxylic acids. Zwitterionic surfactants based on alkyl imidazolines which are alkylated with halocarboxylic acids [DE 36 39 752 A1] or acrylic acid [Seifen-Ole-Fette-Wachse, 109, 20 (1983)] are of particular commercial significance. By contrast, the reaction of tertiary amines with 3-chloro-2-hydroxypropane sulfonic acid to amphoteric surfactants of the sulfobetaine type, which is described for example in British patent application GB 1,541,427, is of minor importance.
All the industrial processes mentioned above have the disadvantage that the reaction of the nitrogen component with the alkylating agent is not complete so that the reaction products contain between 0.1 and 3% by weight unreacted starting materials. Since the content of free amines and unused alkylating agent has to be reduced to a low level in order to obtain toxicologically safe products of acceptable odor, there has been no shortage of attempts in the past to achieve this objective through the choice of suitable reaction conditions or by working up the reaction products.
Whereas the content of free amines in the solution of amphoteric or zwitterionic surfactants can be substantially reduced, for example, by using the alkylating agent in excess, carrying out the reaction at pH=8 to 10 [DE 29 26 479 A1], eliminating most of the amine by distillation or destroying the amine by oxidation [DE 20 63 422 A1], heating the surfactant solutions under alkaline conditions, even for several hours, only leads to a slight reduction in the residual content of alkylating agent.
Accordingly, the problem addressed by the present invention was to provide a process by which the residual content of alkylating agent in solutions of amphoteric or zwittionionic surfactants could be reduced to less than 0.01% by weight, based on a solids content of 100% by weight of the surfactant solutions.